(1) Field of the Invention
The present invention relates to a process and an apparatus for disposing of spent NaS cells.
(2) Related Art Statement
The NaS cell is a cell in which sodium and sulfur are placed in opposite sides of an intermediate tube made of a solid electrolyte, respectively. Such NaS cells have been developed as cells for storing a large amount of electric power and as cells for use in electric automobiles. However, since the service life of the NaS cell is considered to be about 10 years, it is anticipated that a huge number of spent NaS cells will come out in the future.
The present inventors have been developing techniques for disposing of spent NaS cells herald of the world. Inventors' past research revealed that in order to recover sodium and sulfur from the interior of the NaS cell, an opening needs to be formed in the NaS cell that has a complicated multi-tubular structure by cutting to recover the sodium. Then, an inner tube needs to be removed from the NaS cell. During the research, it has been revealed that although sodium can flow from the interior of the NaS cell inside an oil vessel in a state such that the cut opening is directed downwardly, the inner tube needs to be pulled out from the NaS cell inside a heating oil vessel filled with a flowing paraffin or the like so as to prevent the sodium from contacting air. Further, it was revealed that when the above opening is cut and formed in the NaS cell inside oil, oxidization of sodium can be effectively prevented. However, even when the inner tube is pulled out after the spent NaS cell having the opening cut is immersed in the oil inside the heating oil vessel in the state that the cut opening is directed downwardly and then sodium is melted and to flow out and into the oil, various problems exist. That is, if the NaS cell is left for a long time, after the formation of the cut opening, in the state that the opening is directed downwardly, sodium inside the inner tube contacts air so that an oxide film is formed at the surface of sodium. Consequently, even if the NaS cell is heated in the heating oil vessel and it is melted, it is difficult for the sodium to flow due to the existence of the oxide film, and it takes a long time to discharge the sodium. It was further revealed that in order to smoothly extract the sodium, it is necessary to remove the oxidization film.